The present invention relates to an analogue display electronic timepiece comprising a stepping motor having a rotation detecting means, and more particularly to a driving circuit which enables the stepping motor to operate the rotation detecting means stably even if the voltage and internal resistance of the timepiece power source vary.
Recently, a method to detect rotation of a rotor by some means and feedback to a driving circuit have been invented and put into a practical use. After driving pulses are applied to the stepping motor an induced voltage waveform generated in the motor coil by a free movement of the rotor varies according to rotational conditions of the rotor. This technique for detecting the rotation of the rotor taking advantage of the variation of the motor coil voltage waveform will be illustrated.
FIGS. 1A and 1B respectively show a perspective view of an embodiment of a stepping motor and a driving voltage waveform applied to motor coil 3.
FIG. 2 shows a driving circuit and a rotation detecting circuit of the stepping motor.
FIG. 3 shows a voltage waveform induced at a terminal 12 of a detection resistance in case a closed loop 11 is composed of a circuit path 11 after the stepping motor is driven through a circuit path 10 by controlling a gate.
A waveform (a) shows a voltage waveform in the case the rotor rotates normally and a waveform (b) shows a voltage waveform in the case the rotor does not rotate. The rotation and non-rotation of the rotor is easily discriminated by detecting whether the voltage reaches a fixed value or not, electrically. If batteries of large voltage variation such as a lithium battery and a secondary battery are used as a power source of the electronic timepiece having the above mentioned rotation detecting means, the driving power of the stepping motor varies and as a result the movement of the rotor after the stepping motor is driven is influenced.
FIGS. 4A and 4B respectively show voltage characteristic diagrams of the induced voltage waveforms by the detecting circuit versus the voltage value at a peak P when the rotor rotates and the time interval T in which the induced voltage develops. As shown in the diagrams, it is difficult to realize the rotation detecting circuit which detects the induced voltage waveforms stably, especially in the case space is limited like in wrist watches, since the induced voltage waveforms vary to a large degree in accordance with the voltage.